THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 96:9-38, 1995 January

                  SYNTHETIC PROPERTIES OF STARBURST GALAXIES

                                CLAUS LEITHERER
           Space Telescope Science Institute, 3700 San Martin Drive,
                   Baltimore, MD 21218; leitherer@stsci.edu

                                      AND

                              TIMOTHY M. HECKMAN
 Physics and Astronomy Department, Johns Hopkins University, Homewood Campus,
                   Baltimore, MD 21218; heckman@pha.jhu.edu


                                   ABSTRACT

        We present the results of an extensive grid of evolutionary
     synthesis models for populations of massive stars.  The parameter
     space has been chosen to correspond to conditions typically found in
     objects like giant H II regions, H II galaxies, blue compact dwarf
     galaxies, nuclear starbursts, and infrared luminous starburst
     galaxies.  The models are based on the most up-to-date input physics
     for the theory of stellar atmospheres, stellar winds, and stellar
     evolution.
        Observable properties of a population of stars are computed for
     the two limiting cases of an instantaneous burst and a constant
     star-formation rate over a time interval of 3x10^8 yr.  We also
     investigate the effects of star-formation histories which are
     intermediate between those two extreme cases.  Three choices of the
     initial mass functions are studies: a Salpeter and a Miller-Scalo type
     IMF with upper mass limits of 100 Msun, and a Salpeter IMF truncated
     at 30 Msun.  Metallicities of 0.1 Zsun, 0.25 Zsun, Zsun, and 2 Zsun
     are considered.
        The model predictions include the numbers of hot stars, supernova
     rates, colors in the UBVRIJHKL passbands, slopes of the ultraviolet
     continuum observable by IUE, HST, and HUT, equivalent widths of
     hydrogen recombination lines, ionizing fluxes shortward of the H^0,
     He^0, and He^+ ionization edges, the Lyman discontinuity, and mass-
     and energy-deposition rates due to stellar winds and supernovae.
        We discuss the contamination of the stellar ultraviolet, optical,
     and near-infrared continuum by nebular emission.  It is found that
     under typical starburst conditions the nebular continuum is not
     negligible.  Depending on the wavelength, addition of the nebular
     continuum leads to significantly redder or bluer broadband colors than
     obtained from a pure stellar continuum.
        A population of massive stars is not only important in terms of its
     output of radiation but also via its deposition of mechanical energy.
     The output of radiative and mechanical luminosity is compared at
     various starburst epochs.  In a supernova dominated instantaneous
     starburst, the mechanical luminosity can be as large as almost 10% of
     the total radiative luminosity.  This occurs when most massive O stars
     have disappeared, and the synthetic spectrum in the optical and
     near-ultraviolet is dominated by B and A stars.  During this epoch,
     the output of ionizing radiation below 912 A becomes very small, as
     indicated by a very large Lyman discontinuity and a very small ratio
     of ionizing over mechanical luminosity.  We discuss the relevance of
     these results for the interpretation of starburst galaxies, active
     galactic nuclei, and the energetics of the interstellar medium.

     Subject headings: galaxies: evolution -- galaxies: fundamental
                       parameters -- galaxies: starburst -- galaxies:
                       stellar content